The present invention relates generally to transistor switches and, more particularly, to inverting transistor switches.
Transistor switches, or transistors, are well known in the art and are commonly used. Transistors are solid-state electronic devices which are capable of amplification and switching.
Inverting transistors are well known in the art and are commonly used to invert an input signal. Inverting transistors, such as bipolar transistors and metal-oxide semiconductor field effect transistors (MOSFETs), typically contain only three terminals. Specifically, inverting transistors contain an input terminal, an output terminal which is used to connect the transistor to a load and a ground terminal. Contrary to the configuration of noninverting transistors, inverting transistors do not require a fourth terminal connected to a power supply, thereby rendering inverting transistors more desirable than noninverting transistors.
In U.S. Pat. No. 5,134,323 to J. S. Congdon, there is disclosed a noninverting transistor switch having only three terminals. In one embodiment, the three terminal noninverting transistor switch includes first, second and third terminals, a depletion mode field effect transistor (FET) having drain and source electrodes that define a current path in the transistor and are connected to the third and second terminals respectively and a gate electrode for controlling the current flow in the transistor current path. A negative voltage converter having an input electrode, an output electrode and a return electrode has its output electrode coupled to the gate electrode in the FET, its return electrode coupled to the source electrode and its input electrode coupled to the first terminal. In operation, the current flow between the drain and source electrodes will be high when a low signal is applied to the input electrode with respect to the source electrode and will be lower when a higher signal is applied to the input electrode with respect to the source electrode. In another embodiment, the noninverting switch having only three terminals includes first, second and third transistors, wherein the third transistor is coupled through circuitry to the first terminal of the noninverting transistor switch to provide a controlled amount of positive feedback which results in hysteresis or "Schmitt-trigger" like variation of the first terminal input threshold voltage.
Three terminal inverting transistors typically experience a notable drawback. Specifically, a noisy input signal which approaches the threshold voltage for a three terminal inverting transistor can often cause the transistor to experience unwanted state changes or even to remain in between an on switching state and an off switching state, this in-between condition being prone to oscillation and high power dissipation.
Accordingly, switching devices which experience hysteresis are desirable. Hysteretic switches reduce the problem of a noisy input signal causing the switching device to experience unwanted state conditions by using two different threshold voltages. Switching devices which experience hysteresis typically use a high threshold voltage, commonly referred to as the rising or positive threshold voltage, to switch the device during low-to-high input signal transitions and a lower threshold voltage, commonly referred to as the falling or negative threshold voltage, to switch the device during high-to-low input signal transitions.
Schmitt triggers are one well-known type of hysteretic switching device. A Schmitt trigger is a form of a bistable multivibrator and is often used in applications where square waves with a constant amplitude are needed or where sine waves require conversion to square waves. Due to its hysteretic properties, Schmitt triggers are effectively immune to a noisy input signal as long as the peak-to-peak amplitude of the input noise signal is less than the difference between the rising and falling threshold voltages for the device.
In use, a Schmitt trigger functions in the following manner. The Schmitt trigger output voltage remains low until the input signal voltage crosses the rising threshold voltage for the device. Once the input signal voltage crosses the rising threshold voltage for the device, the Schmitt trigger is actuated and the output voltage abruptly rises. Once the input signal voltage falls below the falling threshold voltage for the device, the Schmitt trigger produces an output voltage which drops to a low voltage state almost instantly.
Schmitt triggers are undesirable because the device requires four terminals as opposed to inverting transistors which require only three terminals. In addition to an input terminal, an output terminal and a ground terminal, Schmitt triggers require a power supply terminal, which is undesirable.
Silicon controlled rectifiers (SCR) are four-layer unidirectional devices for bistable switching. A silicon controlled rectifier is essentially a rectifier diode which additionally comprises a control element.
It should be noted that silicon controlled rectifiers are inverting, experience hysteresis and comprise only three terminals, which is desirable.
However, silicon controlled rectifiers experience a notable drawback. Specifically, silicon controlled rectifiers experience output signal actuated hysteresis, which is less desirable than switching devices which experience input signal actuated hysteresis, such as Schmitt triggers. In particular, silicon controlled rectifiers experience output current actuated hysteresis which is load dependent, and therefore highly undesirable.
In use, a silicon controlled rectifier functions in the following manner. The silicon controlled rectifier remains off until the input signal voltage crosses the rising threshold for the device. Once the input signal voltage crosses the rising threshold voltage for the device, the silicon controlled rectifier is actuated and the output current abruptly rises. However, when the input signal voltage falls even below ground, the silicon controlled rectifier does not experience a drop to zero in output current. Unlike the Schmitt trigger, the silicon controlled rectifier can not be said to have a falling input threshold voltage. To the contrary, during turn-off, silicon controlled rectifiers are output signal dependent because the output current must be brought nearly to zero for the silicon controlled rectifier to turn off.
Electromechanical relay circuits are electromechanical coil and contact devices which control power distributed to a load by energizing an isolated input circuit. Electromechanical relay circuits can be built using electrically isolated input and output circuits. In use, an input signal energizes an electromagnet that attracts a hinged and spring-loaded element commonly referred to as an armature. Output contacts, attached to but insulated from the armature, are opened or closed by the movement of the armature. In the closed position, the contacts apply power to the load. In the open position, the contacts remove power to the load.
It should be noted that electromechanical relay circuits can be connected to be inverting, experience input signal actuated hysteresis and comprise only three terminals, which is highly desirable.
However, inverting electromechanical relay circuits experience a number of alternative drawbacks. Specifically, electromechanical relay circuits require a large amount of input power, are large, are slow and are unreliable, which is highly undesirable.